1. Field of the Invention
The present invention relates to a dispersion-compensating module which improves the transmission quality of large-capacity, high-speed optical transmission systems of WDM (Wavelength Division Multiplexing) type.
2. Related Background Art
A WDM type optical transmission system is a system which transmits a plurality of signal light components within a 1.55-xcexcm wavelength band (1.53 xcexcm to 1.57 xcexcm) by way of an optical fiber transmission line network, and enables large-capacity, high-speed optical communications. This optical transmission system comprises an optical amplifier for optically amplifying a plurality of signal light components, collectively, and the like, in addition to an optical fiber line which is a transmission medium. In such WDM communications, various studies and developments are under way in order to enable communications with further larger capacity and higher speed.
One of important subjects to be studied concerning the optical transmission line is reduction of dispersion in a signal wavelength band. Namely, each signal light component has a certain bandwidth even though it is monochromatic. As a result, when dispersion occurs in the signal wavelength band in the optical transmission line, the signal light components having reached a receiving station by way of the optical transmission line after being sent out from a transmitting station deform their waveforms, thereby deteriorating their reception. Therefore, in the signal wavelength band, it is desirable that the dispersion in the optical transmission line be as small as possible.
However, standard single-mode optical fibers (hereinafter referred to as SMF), having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band, already installed as an optical transmission line have a dispersion of about 16 ps/nm/km in a wavelength band of 1.53 xcexcm to 1.57 xcexcm which is used in WDM communications. Many of already installed optical transmission lines are constituted by such an SMF. Therefore, a dispersion-compensating module is disposed within a repeater in order to compensate for the dispersion of the optical transmission line, while making use of such an already installed optical transmission line.
With respect to the dispersion over the whole length of the optical transmission line to be compensated for, the dispersion-compensating module generates a dispersion having an opposite polarity with substantially the same absolute value. Specifically, the dispersion-compensating module comprises a dispersion-compensating optical fiber having a dispersion with a polarity opposite to that of the dispersion of the optical transmission line (including SMF), and compensates for the dispersion of the optical transmission line (including SMF) by adjusting the dispersion-compensating optical fiber to an appropriate length. Also, for reducing the size of the dispersion-compensating module, it is a common practice to wind the dispersion-compensating optical fiber into a coil having a small diameter.
The inventors have studied conventional dispersion-compensating modules and, as a result, have found out the following problems.
Recently, in optical amplifiers, the width of a signal wavelength band which can collectively be optically amplified has been expanded, and gain deviations in optically amplifiable signal wavelength bands have been reduced in order to improve the versatility thereof. On the other hand, inter-wavelength loss deviations (fluctuations in loss among signal wavelengths) occurring in the optical transmission line in the signal wavelength band are too large to neglect. Also, since inter-wavelength loss deviations in the signal wavelength band occur to a certain extent in a dispersion-compensating optical fiber as well, it is necessary to improve loss deviations in the whole transmission line including the conventional dispersion-compensating modules.
When a plurality of stages of conventional dispersion-compensating modules and optical amplifiers are disposed in such an optical transmission line; even if a plurality of signal light components sent out from a transmitting station exhibit no inter-wavelength optical power deviation at the time when sent out, they will generate an optical power deviation, due to inter-wavelength loss deviations, while they are propagating through the optical transmission line and conventional dispersion-compensating modules, and the optical power deviation is expanded by optical amplifiers having a small gain deviation. Since inter-wavelength optical power deviations are accumulated in the plurality of signal light components reaching the receiving station, a part of the signal light components may become weak, thereby generating reception errors.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide a dispersion-compensating module having a structure which compensates for the dispersion of optical transmission lines in a 1.55-xcexcm wavelength band (1.53 xcexcm to 1.57 xcexcm) and adjusts loss fluctuations among signal wavelengths into an appropriate range.
The dispersion-compensating module according to the present invention is generally installed between repeaters together with an optical amplifier, whereas the object to be compensated for thereby is an SMF which is laid between a transmitting station and a receiving station, between repeater stations, between the transmitting station and a repeater station, or between a repeater station and the receiving station. For being installed on an already installed optical fiber transmission line and constituting a part of the line, the dispersion-compensating module comprises an input end for capturing signal lights propagating through the optical fiber transmission line and an output end for sending out the signal lights into the optical fiber transmission line, has a positive loss slope in a 1.55-xcexcm wavelength band, and further comprises a structure for allowing the dispersion generated in a predetermined length of the optical fiber transmission line to be compensated for and loss deviations among individual signal wavelengths to be adjusted into an appropriate range.
Specifically, the dispersion-compensating module according to the present invention comprises a dispersion-compensating device and a loss-equalizing device. The dispersion-compensating device compensates for the dispersion of the above-mentioned optical fiber transmission line in the 1.55-xcexcm wavelength band. Also, for compensating for the wavelength dependence of loss in at least the above-mentioned optical fiber transmission line and dispersion-compensating device, the loss-equalizing device adjusts the total loss slope of the optical fiber transmission line including the dispersion-compensating module such that the loss deviations among individual signal wavelengths in the 1.55-xcexcm wavelength band caused by propagation in the optical fiber transmission line and dispersion-compensating device falls within the appropriate range.
In this specification, xe2x80x9closs slopexe2x80x9d refers to the gradient of a graph indicating the wavelength dependence of transmission loss. Also, the above-mentioned optical fiber transmission line is an SMF having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band; and, letting L be the length of the above-mentioned SMF, and Cr be the absolute value of a permissible manufacturing error, the total loss slope (dB/nm) of the dispersion-compensating module in the 1.55-xcexcm wavelength band is greater than 0 but not greater than 0.000175xc3x97L+xcex1. In general, when an SMF having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band is employed as an optical fiber transmission line, the loss slope per unit length of the SMF is about xe2x88x920.000175 (dB/nrn/km=dB/(nmxc2x7km)). Therefore, the total loss slope of the dispersion-compensating module is ideally +0.000175xc3x97L when an SMF having a length of L is concerned. In practice, however, since the manufacturing error xcex1 cannot be neglected, the loss slope (dB/nm) of the dispersion-compensating module in the 1.55-xcexcm wavelength band is greater than 0 but not greater than 0.000175xc3x97L+xcex1. The loss-equalizing device controls the loss slope of the whole modules such that the total loss slope of the optical fiber line, which includes the dispersion-compensating device in the module, falls within an appropriate range.
The loss-equalizing device includes an optical fiber, comprising a core region doped with a transition metal element and a cladding region disposed at an outer periphery of the core region, in which a single mode is secured in the 1.55-xcexcm wavelength band. The transition metal element preferably include Cr or Co, and the amount of compensation of loss in the loss-equalizing device can be adjusted when the kind of the transition metal element and the amount of addition thereof are appropriately regulated.
The above-mentioned loss-equalizing device may include an optical fiber formed with a long-period grating in which a propagation mode and a radiation mode are coupled to each other. Alternatively, the above-mentioned dispersion-compensating device may be an optical device having, as the above-mentioned loss-equalizing device, a long-period grating in which a propagation mode and a radiation mode are coupled. to each other. In each of these configurations, the long-period grating, which functions as the loss-equalizing device, enables loss deviations among individual signal wavelengths to be adjusted in the whole optical transmission line without increasing the transmission loss of the whole dispersion-compensating module. In particular, in the configuration in which the long-period grating, which functions as the loss-equalizing device, is formed in the optical fiber functioning as the dispersion-compensating device, the dispersion-compensating device does not have a connecting portion which may yield loss. Consequently, it is not necessary to take account of influences of transmission loss in the connecting portion, whereby loss fluctuations among individual wavelengths can be adjusted more easily. Here, as explicitly shown in U.S. Pat. No. 5,703,978, the long-period grating is a grating which induces coupling (mode coupling) between a core mode and a cladding mode which propagate through an optical fiber, and is clearly distinguished from a short-period grating for reflecting light centered at a predetermined wavelength. Also, in the long-period grating, for yielding a strong power conversion from the core mode to the cladding mode, the grating period (pitch) is set such that the optical path difference between the core mode and the cladding mode becomes 2xcfx80. As a consequence, since the long-period grating acts so as to couple the core mode to the cladding mode, the core mode attenuates over a narrow band centered at a predetermined wavelength (hereinafter referred to as xe2x80x9closs wavelengthxe2x80x9d).
The above-mentioned loss-equalizing device can also be realized by fusion-splicing respective end portions of a pair of optical fibers by fusion. In this case, the fused portion of the pair of optical fibers functions as the loss-equalizing device.
Preferably, the optical axes of the pair of optical fibers are shifted from each other in the fused portion. This fused portion can also be realized by fusion-splicing the pair of optical fibers while their core regions are bent. Also, when a pair of optical fibers each having a core region with an outside diameter expanding toward the opposed portion is fusion-spliced to each other, the fused portion can function as the loss-equalizing device.
Further, the loss-equalizing device may include a fiber coupler, or an optical fiber bent at one or more parts thereof. A desirable loss wavelength characteristic can be obtained in each of these cases.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.